Synthetic aperture radar imaging apparatus and methods

ABSTRACT

A synthetic aperture radar (SAR) is operable in an interrogation mode and in a self-imaging mode, the self-imaging mode entered in response to determining a response to interrogation pulses have been received from a ground terminal and position information specifying a ground location has been received from the ground terminal. A ground terminal is operable to receive interrogation pulses transmitted by a SAR, transmit responses, and transmit position information to cause the SAR to enter a self-imaging mode. The ground terminal receives first and subsequent pulses from the SAR where subsequent pulses include backscatter and are encoded. The ground terminal generates a range line by range compression.

BACKGROUND Technical Field

The present application relates generally to synthetic aperture radar(SAR) and, more particularly, to operating modes suitable forsituational awareness.

Description of the Related Art

A synthetic aperture radar (SAR) is an imaging radar. The SAR exploitsthe relative motion of the radar and a target of interest to obtain highazimuthal resolution. High range resolution can be achieved using pulsecompression techniques. The SAR is typically flown on an aircraft, aspacecraft, unmanned aerial vehicle (UAV) such as a drone, or anothersuitable platform. The target of interest is typically on the ground,and can be a point target or a distributed target. The SAR can be acomponent of a SAR imaging system, the system also including at leastone of data processing and data distribution components.

In conventional operation of the SAR imaging system, the system istasked to obtain images of a target or a swath. Data is collectedon-board the platform. In the case of a spaceborne SAR, the data iscollected on-board the spacecraft, and either processed on-board thespacecraft and downlinked to the ground, or downlinked and processed onthe ground to generate the images. The images are distributed to theuser, typically via a network.

BRIEF SUMMARY

A method of operation of a synthetic aperture radar (SAR) comprising atransceiver and a hardware data processor may be summarized asincluding: causing by the data processor the SAR to enter aninterrogation mode; transmitting by the transceiver one or moreinterrogation pulses to the ground; determining by the data processor ifa response to the one or more interrogation pulses has been receivedfrom a ground terminal; determining by the data processor if positioninformation specifying a ground location has been received from theground terminal; upon determining by the data processor a response tothe one or more interrogation pulses has been received from the groundterminal, and position information specifying a ground location has beenreceived from the ground terminal, causing by the data processor the SARto enter a self-imaging mode; transmitting by the transceiver a firsttransmitted pulse to the ground; receiving by the transceiver a firstreceived pulse that includes the first transmitted pulse backscatteredfrom the ground; encoding by the data processor the first received pulseto generate a subsequent transmitted pulse; and transmitting by thetransceiver the subsequent transmitted pulse to the ground.

Encoding the first received pulse by the data processor to generate asubsequent transmitted pulse may include modulating by the dataprocessor the first received pulse by at least one of phase-shift keying(PSK) or amplitude and phase-shift keying (APSK) to generate a modulatedfirst received pulse; and convolutionally encoding by the data processorthe modulated first received pulse to generate an encoded first receivedpulse. Encoding by the data processor the first received pulse togenerate a subsequent transmitted pulse may further include encryptingby the data processor the encoded first received pulse to generate anencrypted subsequent transmitted pulse.

In any of the above described implementations, transmitting one or moreinterrogation pulses to the ground by the transceiver may includetransmitting by the transceiver one or more pulses in a broad-beam modeof the SAR. Transmitting by the transceiver one or more interrogationpulses in a broad-beam mode of the SAR may include transmitting by thetransceiver one or more pulses at a pulse repetition frequency in arange of 0.5 Hz to 50 Hz.

The method of operation of a SAR may be summarized as further including:receiving a subsequent received pulse by the transceiver, the subsequentreceived pulse which includes the subsequent transmitted pulsebackscattered from the ground; encoding by the data processor thesubsequent received pulse to generate a further subsequent transmittedpulse; and transmitting by the transceiver the further subsequenttransmitted pulse to the ground.

The method of operation of a SAR may be summarized as further including:transmitting by the transceiver a second transmitted pulse to the groundbefore transmitting by the transceiver the subsequent transmitted pulseto the ground, wherein the transmitting of the subsequent transmittedpulse does not consecutively follow the transmitting of the firsttransmitted pulse.

A method of generating by a ground terminal a synthetic aperture radar(SAR) image of the ground, the ground terminal comprising a transceiver,a data store, and a hardware data processor, may be summarized asincluding: receiving by the transceiver an interrogation pulsetransmitted by a SAR; transmitting by the transceiver a response to theinterrogation pulse; transmitting by the transceiver positioninformation specifying a ground location; receiving by the transceiveran n^(th) transmitted pulse transmitted by the SAR; storing the n^(th)transmitted pulse in the data store; receiving by the transceiver anm^(th) transmitted pulse transmitted by the SAR, wherein m is greaterthan n, and wherein a radar return received by the SAR includes then^(th) transmitted pulse backscattered to the SAR from the ground, andthe m^(th) transmitted pulse transmitted by the SAR includes an encodingof the radar return received by the SAR; storing the m^(th) transmittedpulse in the data store; decoding by the data processor the m^(th)transmitted pulse to regenerate the radar return received by the SAR;and generating a range line by the data processor for the SAR image ofthe ground by range compression of the radar return received by the SARusing the n^(th) transmitted pulse as a reference function.

In some implementations, the first and the subsequent transmitted pulsesare consecutive pulses. In other implementations, the first and thesubsequent transmitted pulses are successive pulses but notconsecutively successive pulses, and there is at least one interveningtransmitted pulse between the first and the subsequent transmittedpulses.

Decoding by the data processor the m^(th) transmitted pulse toregenerate the radar return received by the SAR may include:convolutionally decoding by the data processor the m^(th) transmittedpulse to generate a modulated radar return; and demodulating by the dataprocessor the modulated radar return to regenerate the radar returnreceived by the SAR.

Encoding of the m^(th) transmitted pulse may include an encryption ofthe radar return received by the SAR, and decoding the m^(th)transmitted pulse by the data processor to regenerate the radar returnreceived by the SAR may include: decrypting by the data processor them^(th) transmitted pulse to generate an encoded radar return;convolutionally decoding by the data processor the encoded radar returnto generate a modulated radar return; and demodulating by the dataprocessor the modulated radar return to regenerate the radar returnreceived by the SAR.

In any of the above described implementations of the method ofgenerating by a ground terminal a synthetic aperture radar (SAR) imageof the ground, transmitting by the transceiver position informationspecifying a ground location may include transmitting by the transceiverposition information that includes the ground location of the groundterminal. Transmitting by the transceiver position informationspecifying a ground location may include transmitting by the transceiverposition information that includes the center of a desired image groundarea.

A synthetic aperture radar (SAR) imaging system may be summarized asincluding: a SAR mounted on a SAR platform, the SAR selectively operablein an interrogation mode to transmit a plurality of interrogation pulsesto the ground; a ground terminal comprising a ground terminal antennacommunicatively coupled to a transceiver, the transceivercommunicatively coupled to at least one of a ground terminal data storeand a ground terminal data processor, wherein the ground terminal isselectively operable to receive an interrogation pulse and respond bytransmitting position information specifying a ground location to theSAR, and wherein the SAR imaging system, in response to receivingposition information from the ground terminal, generates a SAR image ofthe ground that includes the ground location specified in the positioninformation received in response to the interrogation pulse.

The SAR platform may be selected from a group consisting of a spaceborneplatform and an airborne platform. The SAR may include: a SAR antennathat in a self-imaging mode of operation transmits a plurality oftransmitted pulses to the ground that includes a first and a subsequenttransmitted pulse, and receives a plurality of received returnsbackscattered from the ground that includes a first and a subsequentreceived return; a SAR transceiver communicatively coupled to the SARantenna, the SAR transceiver communicatively coupled to at least one ofa SAR data store and a SAR data processor, the SAR data storecommunicatively coupled to the SAR data processor, the SAR data storewhich stores the first received return, and the SAR data processor whichencodes the first received return to generate the subsequent transmittedpulse for transmission by the SAR antenna to the ground. The groundterminal transceiver may be selectively operable to receive thesubsequent transmitted pulse, and the ground terminal data processor maydecode the subsequent transmitted pulse to regenerate the first receivedreturn, retrieve a reference function from the ground terminal datastore, and generate a range line by range compression of the firstreceived return using the reference function. The reference function maybe, at least in part, derived from the first transmitted pulse. The SARdata processor may include: at least one of a phase-shift keying (PSK)modulator or an amplitude and phase-shift keying (APSK) modulator whichin operation generates a modulated first received return from the firstreceived return; and a convolutional encoder which in operationgenerates an encoded first received return from the modulated firstreceived return. The SAR data processor may further include an encrypterwhich encrypts the encoded first received return. The ground terminaldata processor may include: a convolutional decoder which decodes thesubsequent transmitted pulse; at least one of a phase-shift keying (PSK)demodulator or an amplitude and phase-shift keying (APSK) demodulatorwhich in operation regenerates the first received return from thedecoded subsequent transmitted pulse.

In any of the above described implementations of the SAR imaging system,the SAR may be selectively operable in a broad-beam mode, and theplurality of interrogation pulses may be transmitted while the SAR isoperating in the broad-beam mode. The plurality of interrogation pulsesmay be transmitted at a pulse repetition frequency in a range of 0.5 Hzto 50 Hz.

A method of operation of a radar comprising a transmitter, a receiver,and a set of processor circuitry, may be summarized as including:transmitting by the transmitter a first pulse; receiving by the receivera radar return that includes the first pulse returned from a target;encoding by the set of processor circuitry the radar return to generatea subsequent pulse; and transmitting by the transmitter the subsequentpulse.

In some implementations, the first and the subsequent pulses areconsecutive pulses transmitted by the transmitter. In otherimplementations, the first and the subsequent pulses are not consecutivepulses, and there is at least one intervening transmitted pulse betweenthe first and the subsequent pulses.

Receiving by the receiver a radar return that includes the first pulsereturned from a target may include receiving by the receiver a radarreturn that includes the first pulse backscattered from a target on asurface of the Earth. The radar may be operable to image the target onthe surface of the Earth from at least one of an airborne platform or aspaceborne platform. The radar may include a transceiver that includesthe transmitter and the receiver.

Encoding the radar return by the set of processor circuitry to generatea subsequent pulse may include: sampling the radar return to generate asampled radar return; modulating by the set of processor circuitry thesampled radar return by at least one of phase-shift keying (PSK) oramplitude and phase-shift keying (APSK) to generate a modulated radarreturn; and convolutionally encoding by the set of processor circuitrythe modulated radar return to generate an encoded radar return. Encodingthe radar return by the processor circuitry to generate a subsequentpulse further may include encrypting by the set of processor circuitrythe encoded radar return to generate an encrypted subsequent pulse.

A method of operation of a synthetic aperture radar comprising atransmitter, a receiver, and a set of processor circuitry may besummarized as including: transmitting by the transmitter a first pulse;receiving by the receiver a radar return that includes the first pulsereturned from a target; encoding by the set of processor circuitry theradar return to generate a subsequent pulse; and transmitting by thetransmitter the subsequent pulse.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not necessarily drawn to scale, and some ofthese elements may be arbitrarily enlarged and positioned to improvedrawing legibility. Further, the particular shapes of the elements asdrawn, are not necessarily intended to convey any information regardingthe actual shape of the particular elements, and may have been solelyselected for ease of recognition in the drawings.

FIG. 1A is a schematic diagram illustrating the illumination geometry ofan example embodiment of a SAR imaging system in accordance with thepresent systems, devices, methods, and articles.

FIG. 1B is a schematic diagram illustrating the illumination geometry ofanother example embodiment of a SAR imaging system in accordance withthe present systems, devices, methods, and articles.

FIG. 2 is a block diagram illustrating an example embodiment of a SARimaging system in accordance with the present systems, devices, methods,and articles.

FIG. 3 is a schematic diagram illustrating operation of an exampleembodiment of a SAR imaging system in accordance with the presentsystems, devices, methods, and articles.

FIG. 4 is a timing diagram illustrating an example timing sequence foroperation of a SAR imaging system in accordance with the presentsystems, devices, methods, and articles.

FIG. 5 is a flow chart illustrating a method of operation of a SARimaging system in accordance with the present systems, devices, methods,and articles.

FIG. 6 is a timing diagram illustrating an example timing sequence fortransmitted and received pulses during operation of a SAR imaging systemin accordance with the present systems, devices, methods, and articles.

FIG. 7 is a flow chart illustrating an example method of encoding atransmitted pulse of a SAR imaging system in accordance with the presentsystems, devices, methods, and articles.

FIG. 8 is a flow chart illustrating another example method of encoding atransmitted pulse of a SAR imaging system in accordance with the presentsystems, devices, methods, and articles.

FIG. 9 is a flow chart illustrating a method of range compressingreceived pulses by a ground terminal in accordance with the presentsystems, devices, methods, and articles.

FIG. 10 is a flow chart illustrating another method of range-compressingreceived pulses by a ground terminal in accordance with the presentsystems, devices, methods, and articles.

FIG. 11 is a flow chart illustrating an example method of forming aself-image by a ground terminal in accordance with the present systems,devices, methods, and articles.

FIG. 12 is a flow chart illustrating another example method of encodinga transmitted pulse of a radar in accordance with the present systems,devices, methods, and articles.

FIG. 13 is a flow chart illustrating another example method ofrange-compressing received pulses by a ground terminal of a radar inaccordance with the present systems, devices, methods, and articles.

DETAILED DESCRIPTION

The technology described in this application is a SAR mode forapplications requiring real-time or near real-time situational awarenesson demand, such as search and rescue. It is particularly suitable forcircumstances where a person in the field needs to know what is in theimmediate vicinity, say, in a local 5 km to 10 km square area. Thetechnology is suitable for a SAR imaging system mounted on a spacecraft,an aircraft or a drone, for example. The person in the field requires aground terminal for receiving signals from the SAR and for transmittingsignals to the SAR.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its broadest sense, that is as meaning “and/or”unless the content clearly dictates otherwise.

The Abstract of the Disclosure and headings provided herein are forconvenience only and do not interpret the scope or meaning of theembodiments.

FIG. 1A is a schematic diagram illustrating the illumination geometry ofan example embodiment of a SAR imaging system in accordance with thepresent systems, devices, methods, and articles. The SAR imaging systemof FIG. 1A comprises a spaceborne platform 110 a and a ground terminal120. Spaceborne platform 110 a comprises a synthetic aperture radar (notshown in FIG. 1A). The elements of the synthetic aperture radar (SAR)are described in reference to FIG. 2 (below).

Spaceborne platform 110 a can be, for example, a satellite, aspacecraft, or a space station. In some implementations, spaceborneplatform 110 a can be replaced by an aircraft or an unmanned aircraftsuch as a drone, for example (see FIG. 1B). In some implementations,ground terminal 120 is mobile. In other implementations, ground terminal120 is fixed. In some implementations, ground terminal 120 isman-portable.

Spaceborne platform 110 a flies along trajectory 130. Dashed line 140indicates the ground track of spaceborne platform 110 a. Line 150 andline 155 indicate the near-side and the far-side of a swath,respectively. Shaded region 160 represents a main lobe of an SAR antennabeam pattern on the ground. As spaceborne platform 110 a flies alongtrajectory 130, ground terminal 120 will first enter and then leave theSAR antenna beam, as represented by shaded region 160.

FIG. 1B is a schematic diagram illustrating the illumination geometry ofanother example embodiment of a SAR imaging system in accordance withthe present systems, devices, methods, and articles using an airborneplatform 110 b instead of spaceborne platform 110 a. Airborne platform110 b can be, for example, an aircraft, an unmanned aircraft, or adrone.

FIG. 2 is a block diagram illustrating an example embodiment of SARimaging system 200 in accordance with the present systems, devices,methods, and articles. SAR imaging system 200 comprises syntheticaperture radar (SAR) 210 and ground terminal 220. SAR 210 can be mountedon an airborne or spaceborne SAR platform such as an aircraft, drone,satellite or space station. SAR 210 can be flown on a satellite such asspaceborne platform 110 a of FIG. 1A.

SAR 210 comprises one or more antenna 212, transceiver 214,nontransitory SAR data storage media 216, and SAR data processor 218(e.g., hardware circuitry). Antenna 212 is bi-directionallycommunicatively coupled to transceiver 214. Transceiver 214 isbi-directionally communicatively coupled to data storage 216 and dataprocessor 218. Data storage 216 is bi-directionally communicativelycoupled to data processor 218.

Data storage 216 can take the form of one or more computer- orprocessor-readable memories or storage media, for instance volatilememory (e.g., RAM), nonvolatile memory (e.g., ROM, FLASH, EEPROM), orspinning media (e.g., magnetic disk, optical disk) with associatedreaders and/or writers.

Data processor 218 can comprise one or more data processing elementssuch as a modulator, an encoder, an encrypter and the like. Dataprocessor 218 can also comprise one or more control elements such as acontroller to determine when to switch modes of operation, to commandthe SAR to switch operation and to synchronize operations in each mode.

Data processor 218 can take the form of one or more circuits orcircuitry or hardware, for instance one or more microprocessors (singleor multicore), central processor units (CPUs), digital signal processors(DSPs), graphic processing units (GPUs), application specific integratedcircuits (ASICs), programmable gate arrays (PGAs), or programmable logicunits (PLUs).

Ground terminal 220 comprises antenna 222, transceiver 224, terminaldata storage 226, terminal data processor 218, and terminal display 219.Antenna 222 is bi-directionally communicatively coupled to transceiver224, and transceiver 224 is communicatively coupled to data storage 226.Data received at antenna 222 can be communicated to transceiver 224, andstored in data storage 226. Transceiver 224 can transmit data viaantenna 222. Data storage 226 is bi-directionally communicativelycoupled to data processor 228. Display 229 can receive data for displayfrom data storage 226 and data processor 228.

Data storage 226 can take the form of one or more computer- orprocessor-readable memories or storage media, for instance volatilememory (e.g., RAM), nonvolatile memory (e.g., ROM, FLASH, EEPROM), orspinning media (e.g., magnetic disk, optical disk) with associatedreaders and/or writers. Data processor 228 can take the form of one ormore circuits or circuitry or hardware, for instance one or moremicroprocessors (single or multicore), central processor units (CPUs),digital signal processors (DSPs), graphic processing units (GPUs),application specific integrated circuits (ASICs), programmable gatearrays (PGAs), or programmable logic units (PLUs).

In some implementations, SAR 210 has a sufficiently large transmit powerthat antenna 222 of ground terminal 220 can be relatively small. Forexample, antenna 222 of ground terminal 220 can be small enough thatground terminal 220 is man-portable, and consequently easily deployed,such as in search and rescue operations. Terminal data processor 218typically has sufficient processing power to generate SAR images by SARdata processing in near real-time, for example within an hour ofreception of the raw data by ground terminal 220.

FIG. 3 illustrates operation of an example embodiment of a SAR imagingsystem in accordance with the present systems, devices, methods, andarticles. The SAR imaging system comprises a SAR and a ground terminal(such as SAR 210 and ground terminal 220 of FIG. 2). The SAR platform(not shown in FIG. 3) overflies region of interest (enclosed by brokenline 310), and beam ground track 320 intersects region of interest 310.Beam ground track 320 enters region of interest 310 at position 312, andleaves region of interest 310 at position 314.

The SAR imaging system transmits interrogation pulses at periodicintervals when the SAR imaging system is at various positions 330A,330B, 330C, and 330D (collectively 330) in its transit, before andduring its overflight of region of interest 310. The interrogationpulses can be generated using a broad-beam mode of the SAR imagingsystem.

In an example implementation, the SAR transmits pulses at a low pulserepetition frequency in the range 0.5 Hz to 50 Hz (e.g., 20 Hz) when inthe broad-beam mode. Each pulse is an indication to the ground terminal,or to a user on the ground, that the SAR is approaching, and ready to becommanded into a self-imaging mode by the ground terminal. The SARantenna beam in the broad-beam mode can be generated, for example, bydecreasing the active antenna area, or by beam shaping, either in rangeor azimuth, or both.

A ground terminal (not shown in FIG. 3) can receive an interrogationpulse transmitted in the broad-beam mode. The ground terminal canrespond by firstly acknowledging receipt of the interrogation pulse, andthen transmitting a set of coordinates to the SAR corresponding to acenter of a desired image ground area 340. In some examples, the set ofcoordinates include the latitude and the longitude of the center ofdesired image ground area 340. In some examples, the center of desiredimage ground area 340 is coincident with the location of the groundterminal. In other examples, the center of desired image ground area 340is selected such that the ground terminal and the desired image groundarea are both within the SAR imaging beam.

In an example implementation, the SAR transmits at L-band (λ=24 cm) andhas along-track antenna dimension D=6 m. In this example, the broad-beammode of the SAR transmits a beam having a cross-track beamwidth (definedas the angle of the half-power points of the main lobe) of θ_(CT)=20°,and an along-track beamwidth λ/D of θ_(AT)=2.3°. At a range ofapproximately 450 km, the along-track beamwidth on the ground isapproximately 18 km.

The pulses transmitted by the SAR in the broad-beam mode can benarrow-band pulses, selected so that there is sufficient link margin forthe ground terminal (such as ground terminal 220 of FIG. 2) to receivethe pulses. The ground terminal can receive an interrogation pulse via asidelobe of the SAR beam, for example the first sidelobe or the secondsidelobe of the SAR beam. The first and the second sidelobes of theantenna pattern of the SAR beam are typically approximately 13 dB and 28dB below the mainlobe, respectively. In an example implementation, theground terminal receives one of the interrogation pulses via the firstsidelobe of the SAR beam, providing approximately 2.5 seconds before thesatellite is able to image desired image ground area 340, such as aregion centered on the ground terminal's coordinates.

Upon receiving the set of coordinates from the ground terminal, the SARswitches to a narrow-beam mode (the narrow-beam mode having a beamwidthnarrower than the broad-beam mode), and starts data transmission andcollection for imaging.

FIG. 4 is a timing diagram illustrated an example timing sequence foroperation of a SAR imaging system in accordance with the presentsystems, devices, methods, and articles. FIG. 4 shows three synchronizedtiming graphs 410, 420, and 430. Graph 410 shows a plot of along-trackbeamwidth θ_(AT) as a function of time t. Graph 420 shows a plot ofcross-track beamwidth θ_(CT) as a function of time t. Graph 430 shows aplot of incidence angle of the beam center θ_(i) as a function of timet. The incidence angle of the beam center θ_(i) is related to thecross-track position of the beam center on the ground.

At time t₁, the SAR imaging system switches to an interrogation mode andstarts to transmit interrogation pulses (such as pulses 330 of FIG. 3).In some implementations, along-track beamwidth θ_(AT) is unchanged, andhas a value of θ_(AT1) both before and after time t₁. In otherimplementations, along-track beamwidth θ_(AT) can be broadened and(optionally) steered, and changes value from θ_(AT1) to θ_(AT2) at timet₁.

Cross-track beamwidth θ_(CT) undergoes a stepwise increase as the SARimaging system changes from a narrow-beam mode to a broad-beam mode. Attime t₁, cross-track beamwidth θ_(CT) changes from a value of θ_(CT1) toa value of θ_(CT2).

At time t₁, incidence angle of the beam center θ_(i) can change from avalue of θ_(i1) to a value of θ_(i2).

At time t₂, the SAR imaging system receives a response from the groundterminal. The along-track and cross-track beamwidths, and the incidenceangle of the beam center remain unchanged.

At time t₃, the SAR imaging system receives the coordinates from theground terminal and switches to a self-imaging mode. Along-trackbeamwidth θ_(AT) may change from θ_(AT2) to θ_(AT3) at time t₃. In someexamples, θ_(AT3) can be the same as θ_(AT1).

Cross-track beamwidth θ_(CT) undergoes a stepwise decrease as the SARimaging system changes from a broad-beam mode to a narrow-beam imagingmode. At time t₃, cross-track beamwidth θ_(CT) changes from a value ofθ_(CT2) to a value of θ_(CT3).

At time t₃, incidence angle of the beam center θ_(i) can change from avalue of θ_(i2) to a value of θ_(i3) that accommodates the coordinatesof the ground terminal and the desired image ground area. Incidenceangles of the beam center θ_(i1), θ_(i2), and θ_(i3) are incidenceangles within the incidence angle capability of the SAR.

At time t₄, the SAR imaging system begins imaging of the desired imageground area, centered on the coordinates provided by the groundterminal.

In an example implementation, values of along-track beamwidth,cross-track beamwidth, and incidence angle at beam center are asfollows:

θ_(AT1)=2.3°, θ_(AT2)=2.3°

θ_(CT1)=8°, θ_(CT2)=20°, θ_(CT3)=8°

θ_(i1)=25°, θ_(i2)=30°, θ_(i3)=35°

The first transmitted pulse in the narrow-beam mode is a linear FMchirp. The ground terminal receives and stores the transmitted pulse foruse as a reference function.

The transmitted pulse is backscattered and received at the SAR. The SARdigitizes the received pulse, encodes it, and (optionally) encrypts it.The received data is used as the next transmitted pulse. It isnoise-like, and has a desirable autocorrelation function.

The ground terminal receives the second transmitted pulse, which is theencrypted encoded return from the first pulse. The ground terminaldecrypts the second transmitted pulse, decodes the decrypted secondtransmitted pulse, and processes the decoded and decrypted secondtransmitted pulse with the linear FM reference pulse that was capturedfrom the first transmission. The ground terminal now has the first rangeline of a SAR image, and the reference function for the next range line,where the reference function for the next range line is derived from thesecond transmitted pulse.

The second transmitted pulse (the encrypted encoded return from thefirst transmitted pulse) is backscattered and received at the SAR. Asbefore, the SAR digitizes the second transmitted pulse, encodes thesecond transmitted pulse, and (optionally) encrypts the encoded secondtransmitted pulse. Then the SAR uses the new “data” as the next transmitpulse. It too is noise-like, and has a desirable autocorrelationfunction.

The ground terminal receives the encrypted encoded return from thesecond pulse, decrypts the encrypted encoded return, decodes it, andprocesses it with the encrypted transmission that was captured from thesecond transmission. The ground terminal now has the second range lineas well as the reference function for the next range line derived fromthe third transmitted pulse. This process is repeated for all the rangelines needed to form the SAR image.

More detail is provided with reference to FIGS. 5 to 11.

FIG. 5 is a flow chart illustrating a method of operation 500 of a SARimaging system in accordance with the present systems, devices, methods,and articles. Method 500 begins at 505 for example in response to arequest from an operator input or in response to a command from anothersystem.

At 510, the SAR imaging system switches to an interrogation mode, and,at 520, broadcasts an interrogation signal. In some implementations, theinterrogation signal can be a sequence of interrogation pulses (such aspulses 330 of FIG. 3). For example, the interrogation signal can be asequence of pulses generated in a broad-beam mode of the SAR andtransmitted at a PRF of 20 Hz.

At 522, the SAR imaging system determines if a response to theinterrogation signal from a ground terminal has been received. Inresponse to determining that a response to the interrogation signal hasbeen received (YES) at 522, control in method 500 proceeds to 524. Inresponse to determining that a response to the interrogation signal hasnot been received (NO) at 522, control in method 500 returns to 520. Theloop defined by 520 and 522 causes method 500 to wait until a responsefrom a ground terminal to an interrogation signal from the SAR has beenreceived at the SAR, or until the SAR imaging system exits theinterrogation mode, for example upon satisfying a timeout condition.

At 524, the SAR imaging system determines if position information fromthe corresponding ground terminal, such as the center of the desiredimage ground area, has been received. In response to determining thatposition information has been received (YES) at 524, control in method500 proceeds to 530. In response to determining that positioninformation has not been received (NO) at 524, control in method 500returns to 524. The loop defined by 524 causes method 500 to wait untilposition information for the responding ground terminal has beenreceived, or until the SAR imaging system exits the interrogation mode,for example upon satisfying a timeout condition.

At 530, the SAR imaging system switches to a self-imaging mode andinitiates generation of a self-image. At 535, the SAR imaging systemdetermines the self-image is complete. In response to determining thatthe image is complete (YES) at 535, control in method 500 proceeds to540. In response to determining that the image is not complete (NO) at535, control in method 500 returns to 535. The loop defined by 535causes method 500 to wait until the self-image is complete, or until theSAR imaging system terminates the generation of the self-image, forexample upon satisfying a timeout condition and/or exiting theself-imaging mode.

At 540, the SAR imaging system switches to a normal mode of operation orback to interrogation mode, depending on the SAR imaging system'stasking plan, or in response to a request from an operator or a commandfrom another system, and method 500 terminates at 550, for example untilcalled or invoked again.

FIG. 6 is a timing diagram illustrating an example timing sequence 600for transmitted and received pulses during operation of a SAR imagingsystem in accordance with the present systems, devices, methods, andarticles. Timing sequence 600 comprises transmit (TX) timing sequence610, and receive (RX) timing sequence 620.

FIG. 6 illustrates a sequence of three transmitted pulses 612, 614, and616, and three corresponding received pulses 622, 624, and 626. Inoperation, the SAR imaging system can transmit and receive any suitablenumber of pulses. In some implementations, one or more of thetransmitted pulses 612, 614, 616, etc. are linear frequency-modulated(FM) pulses. Each received pulse (622, 624, 626, etc.) is asuperposition of reflections of a corresponding transmitted pulse (612,614, 616, etc. respectively). The reflections are from targets on theground. Received pulses 622, 624, 626, etc. are typically noise-like.

FIG. 7 is a flow chart illustrating an example method 700 of encoding atransmitted pulse of a SAR imaging system in accordance with the presentsystems, devices, methods, and articles. Method 700 begins at 705 forexample in response to a request from an operator or in response to acommand from another system.

At 710, the SAR imaging system transmits a first linear FM pulse (suchas transmitted pulse 612 of FIG. 6), and, at 720, receives the firstpulse (such as received pulse 622 of FIG. 6) reflected from the ground.

At 730, the SAR imaging system samples the first received pulse, forexample to generate 8-bit in-phase (I) and quadrature (Q) samples, and,at 735, generates an uncompressed range line. The uncompressed rangeline can typically be a quasi-stationary Gaussian signal, for examplewhere the pulse has been backscattered from a distributed target.

In a first encoding operation, at 740, the SAR imaging system encodesthe data, for example to 8PSK (Phase-Shift Keying using 8 phases). PSKis a digital modulation technique in which a base signal isphase-modulated to represent the data. In a second encoding operation,at 745, the SAR imaging system convolutionally encodes and (optionally)encrypts the 8PSK-encoded data. Other encoding schemes can be used.

For example, 16PSK modulation can be used. The selected encoding schemecan depend, for example, on the size of the power of the SAR antenna andthe antenna gain-to-noise-temperature (G/T) of the ground terminalantenna. A 16PSK scheme may be more suitable for a high power SARtransmitter and/or a high ground terminal antenna G/T. An 8PSK schememay be more suitable for a lower power SAR transmitter and/or a lowerground terminal antenna G/T. If the transmitter has amplitude modulationcapability, then the transmitted pulse can be encoded, at least in part,using amplitude modulation. Amplitude and phase-shift keying (APSK) canbe used as a modulation scheme. In an example scheme, 8PSK is augmentedby four additional vectors at half-amplitude at phase angles of 0°, 90°,180°, and 270°.

The goal of encoding the data is usually to achieve a uniform powerspectral density within the SAR operating bandwidth. A uniform powerspectral density is typically associated with pseudo-random noise, whichis desirable for the encoded data because the autocorrelation function(also known as the range point spread function) will be close to anideal sin(x)/x curve for pseudo-random noise.

At 750, the SAR imaging system transmits the encoded first receivedpulse as a second transmitted pulse. Method 700 terminates at 760, forexample until called or invoked again.

FIG. 8 is a flow chart illustrating another example method 800 ofencoding a transmitted pulse of a SAR imaging system in accordance withthe present systems, devices, methods, and articles. Method 800 beginsat 805 for example in response to a request from an operator or inresponse to a command from another system.

At 810, the SAR imaging system transmits the n^(th) pulse, for exampleusing PR 8PSK encoding as described above, and, at 820, receives then^(th) pulse reflected from the ground.

At 830, the SAR imaging system samples the n^(th) received pulse, forexample to generate 8-bit in-phase (I) and quadrature (Q) samples, and,at 835, generates an uncompressed range line.

In a first encoding operation, at 840, the SAR imaging system encodesthe data, for example to 8PSK (Phase-Shift Keying using 8 phases).

PSK is a digital modulation technique in which a base signal isphase-modulated to represent the data. In a second encoding operation,at 845, the SAR imaging system convolutionally encodes and (optionally)encrypts the 8PSK-encoded data.

At 850, the SAR imaging system transmits the encoded n^(th) receivedpulse as the n+1^(th) transmitted pulse. Method 800 terminates at 860,for example until called or invoked again.

FIG. 9 is a flow chart illustrating a method 900 of range compressingreceived pulses by a ground terminal in accordance with the presentsystems, devices, methods, and articles.

At 910, the ground terminal retrieves a copy of TX1, the firsttransmitted pulse. In some implementations, as described above, TX1 is alinear FM pulse. At 920, the ground terminal retrieves a copy of TX2,the second transmitted pulse built from the first received pulse RX1(see for example FIG. 6).

At 930 and 935, the ground terminal decodes and demodulates TX2 in afirst and a second act, respectively. The first act can includedecryption and decoding, and the second act can include demodulation. At940, the ground terminal regenerates RX1, the first received pulse, and,at 950, the ground terminal performs range compression using TX1 andRX1, the replica and the first received pulse, respectively.

At 960, the ground terminal stores the first range line (RX1range-compressed).

FIG. 10 is a flow chart illustrating another method 1000 of rangecompressing received pulses by a ground terminal in accordance with thepresent systems, devices, methods, and articles.

At 1010, the ground terminal retrieves a copy of TX2, the firsttransmitted pulse. In some implementations, as described above, TX2 is atransmitted pulse built from RX1. At 1020, the ground terminal retrievesa copy of TX3, the third transmitted pulse built from the secondreceived pulse RX2 (see for example FIG. 6).

At 1030 and 1035, the ground terminal decodes TX3 in a first and asecond act, respectively. At 1040, the ground terminal regenerates RX2,the second received pulse, and, at 1050, the ground terminal performsrange compression using TX2 and RX2, the replica and the second receivedpulse, respectively.

At 1060, the ground terminal stores the second range line (RX2range-compressed).

FIG. 11 is a flow chart illustrating an example method 1100 of forming aself-image by a ground terminal in accordance with the present systems,devices, methods, and articles. The flow chart in FIG. 11 describes actsperformed by a data processor such as terminal data processor 280 ofFIG. 2.

At 1110, the data processor performs range compression on the firstreceived pulse (RX1). At 1120, the data processor performs rangecompression on the next received pulse. At 1125, the data processordetermines whether there are more range lines to compress. In responseto determining there are more range lines to compress, control of method1100 returns to 1120. In response to determining there are no more rangelines to compress, control of method 1100 proceed to 1130. The loopdefined by 1120 and 1125 causes method 1100 to range compress all thelines needed to form a desired SAR image from the data.

At 1130, the data processor optionally performs range cell migrationcorrection (as indicated by the dashed box). Range cell migrationcorrection may be required depending on the azimuth beamwidth and therange resolution. At 1140, the data processor performs azimuthcompression using the range compressed (and optionally range cellmigration corrected) data.

At 1150, the data processor outputs a SAR image for storage, transferover a network, and/or display on the ground terminal.

Method 1100 is usually referred to as a range-Doppler method forgenerating SAR images. Other approaches can be used that also includebuilding a transmitted pulse from a backscattered and previouslyreceived pulse. Range cell migration correction can be included, asdescribed above, as required by the azimuth beamwidth and the rangeresolution.

In some radars, such as airborne synthetic aperture radars, an echo of atransmitted pulse (e.g., the transmitted pulse backscattered by a targetsuch as the ground) is received directly after the transmitted pulse,and before the next pulse is transmitted. In other radars, such asspaceborne SARs, an echo of the transmitted pulse is received after oneor more subsequent pulses have been transmitted. The intervening pulsesare in flight between the radar and the target—on their way to thetarget or on their way back from the target. The number of interveningpulses can depend on the viewing geometry of the radar.

When there are intervening pulses in flight between a transmitted pulseand its received echo, the systems and methods in the present disclosuredescribe the transmission of an n^(th) pulse, the encoding of the echoof the n^(th) pulse, and the transmission of the encoded echo of then^(th) pulse as the m^(th) transmitted pulse, where m>n. If there are nointervening pulses, then m=n+1.

In some implementations, the radar is a pulse radar. In otherimplementations, the radar is SAR.

FIG. 12 is a flow chart illustrating an example method 1200 of encodinga transmitted pulse of a radar in accordance with the present systems,devices, methods, and articles. Method 1200 begins at 1205 for examplein response to a request from an operator or in response to a commandfrom another system.

At 1210, the radar transmits an n^(th) pulse in a sequence of N pulses.The n^(th) pulse may be a linear FM pulse (such as transmitted pulse 612of FIG. 6), or another suitable pulse such as an unmodulated pulse or apulse that includes encoded information. The encoded information caninclude a received echo from a previously transmitted pulse, forexample.

At 1220, the radar receives the n^(th) pulse reflected from a target.The target can be a point target or a distributed target. The radartransmission can be directed at the ground from an airborne orspaceborne vehicle, for example, and the target can be the ground.

At 1230, the radar samples the received pulse, for example to generatein-phase (I) and quadrature (Q) samples. The I and Q samples can be8-bit samples, for example. In a synthetic aperture radar, the samplescan be used to generate an uncompressed range line. The uncompressedrange line can be a quasi-stationary Gaussian signal, for example wherethe pulse has been backscattered from a distributed target.

The sampled pulse can be encoded. Encoding can be performed in a singleoperation or in multiple operations. In the example illustrated in theFIG. 12, encoding is performed by two operations. In a first encodingoperation, at 1240, the radar encodes the data, for example to 8PSK(Phase-Shift Keying using 8 phases). PSK is a digital modulationtechnique in which a base signal is phase-modulated to represent thedata. In a second encoding operation, at 1245, the radar convolutionallyencodes the data output from the first encoding operation. At 1250, theradar (optionally) encrypts the encoded data output from the secondencoding operation.

Other suitable encoding and/or modulation schemes can be used. Forexample, 16PSK modulation can be used. The selected encoding scheme candepend, for example, on the size of the power of the radar antenna andthe antenna gain-to-noise-temperature (G/T) of a ground terminal antennaused to receive the transmitted pulses. In the case of a syntheticaperture radar, a 16PSK scheme may be more suitable for a high power SARtransmitter and/or a high ground terminal antenna G/T. An 8PSK schememay be more suitable for a lower power SAR transmitter and/or a lowerground terminal antenna G/T. If the transmitter has amplitude modulationcapability, then the transmitted pulse can be encoded, at least in part,using amplitude modulation. Amplitude and phase-shift keying (APSK) canbe used as a modulation scheme. In an example scheme, 8PSK is augmentedby four additional vectors at half-amplitude at phase angles of 0°, 90°,180°, and 270°.

The goal of encoding the data is usually to achieve a uniform powerspectral density within the radar's operating bandwidth. A uniform powerspectral density is typically associated with pseudo-random noise, whichis desirable for the encoded data because the autocorrelation function(also known as the range point spread function) will be close to anideal sin(x)/x curve for pseudo-random noise.

At 1260, the radar generates an m^(th) transmitted pulse based at leastin part on the encoded, and optionally encrypted, received n^(th) pulse.At 1270, the radar transmits the m^(th) transmitted pulse.

Method 1200 terminates at 1280, for example until called or invokedagain.

FIG. 13 is a flow chart illustrating a method 1300 of range-compressingreceived pulses by a ground terminal in accordance with the presentsystems, devices, methods, and articles.

At 1310, the ground terminal retrieves a copy of TXn, the n^(th)transmitted pulse. At 1320, the ground terminal retrieves a copy of TXm,the m^(th) transmitted pulse built from the n^(th) received pulse RXn.

In some implementations, TXm is encrypted, and, at 1325, the groundterminal decrypts TXm. Dashed lines are used in 1325 of FIG. 13 toindicate that decryption is optional. Decoding of TXm can be performedin one or more acts. In the illustrated example of FIG. 13, the groundterminal decodes and/or demodulates TXm in single act 1330. In oneexample implementation, the first act can include decryption anddecoding, and the second act can include demodulation. At 1340, theground terminal regenerates RXn, the n^(th) received pulse, and, at1350, the ground terminal performs range compression using TXn and RXn,the replica and the n^(th) received pulse, respectively.

At 1360, the ground terminal stores a compressed range line (RXnrange-compressed).

While the foregoing description refers, for the most part, to satelliteplatforms for SAR and optical sensors, remotely sensed imagery can beacquired using airborne sensors including, but not limited to, aircraftand drones. The technology described in this disclosure can, forexample, be applied to imagery acquired from sensors on spaceborneplatforms and/or airborne platforms.

The various embodiments described above can be combined to providefurther embodiments. U.S. Provisional Patent Application Ser. No.62/260,063, filed Nov. 25, 2015, and the various patents, applicationsand publications described above are incorporated herein by reference,in their entirety. Aspects of the embodiments can be modified, ifnecessary to employ concepts of the various patents, applications andpublications to provide yet further embodiments.

The foregoing detailed description has, for instance, set forth variousembodiments of the devices and/or processes via the use of blockdiagrams, schematics, and examples. Insofar as such block diagrams,schematics, and examples contain one or more functions and/oroperations, it will be understood by those skilled in the art that eachfunction and/or operation within such block diagrams, flowcharts, orexamples can be implemented, individually and/or collectively, by a widerange of hardware, software, firmware, or virtually any combinationthereof. In one embodiment, the present subject matter may beimplemented via Application Specific Integrated Circuits (ASICs).However, those skilled in the art will recognize that the embodimentsdisclosed herein, in whole or in part, can be equivalently implementedin standard integrated circuits, as one or more computer programsrunning on one or more computers (e.g., as one or more programs runningon one or more computer systems), as one or more programs running on oneor more controllers (e.g., microcontrollers) as one or more programsrunning on one or more processors (e.g., microprocessors), as firmware,or as virtually any combination thereof, and that designing thecircuitry and/or writing the code for the software and or firmware wouldbe well within the skill of one of ordinary skill in the art in light ofthis disclosure.

In addition, those skilled in the art will appreciate that themechanisms of taught herein are capable of being distributed as aprogram product in a variety of forms, and that an illustrativeembodiment applies equally regardless of the particular type of signalbearing media used to actually carry out the distribution. Examples ofsignal bearing media include, but are not limited to, the following:recordable type media such as floppy disks, hard disk drives, CD ROMs,digital tape, and computer memory; and transmission type media such asdigital and analog communication links using TDM or IP basedcommunication links (e.g., packet links).

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A method of operation of a synthetic aperture radar (SAR) comprisinga transceiver and a hardware data processor, the method comprising:causing by the data processor the SAR to enter an interrogation mode,and while in interrogation mode: transmitting by the transceiver one ormore interrogation pulses to the ground; determining by the dataprocessor if a response to the one or more interrogation pulses has beenreceived from a ground terminal; determining by the data processor ifposition information specifying a ground location has been received fromthe ground terminal; upon determining by the data processor a responseto the one or more interrogation pulses has been received from theground terminal, and position information specifying a ground locationhas been received from the ground terminal, causing by the dataprocessor the SAR to enter a self-imaging mode, and while in imagingmode: transmitting by the transceiver a first transmitted imaging pulseto the ground; receiving by the transceiver a first received pulse thatincludes the first transmitted imaging pulse backscattered from theground; encoding by the data processor the first received pulse togenerate a subsequent transmitted imaging pulse; and transmitting by thetransceiver the subsequent transmitted imaging pulse to the ground. 2.The method of claim 1 wherein encoding by the data processor the firstreceived pulse to generate a subsequent transmitted imaging pulseincludes: modulating by the data processor the first received pulse byat least one of phase-shift keying (PSK) or amplitude and phase-shiftkeying (APSK) to generate a modulated first received pulse; andconvolutionally encoding by the data processor the modulated firstreceived pulse to generate an encoded first received pulse.
 3. Themethod of claim 2 wherein encoding by the data processor the firstreceived pulse to generate a subsequent transmitted imaging pulsefurther includes encrypting by the data processor the encoded firstreceived pulse to generate an encrypted subsequent transmitted imagingpulse.
 4. The method of claim 1 wherein transmitting by the transceiverone or more interrogation pulses to the ground includes transmitting bythe transceiver one or more pulses in a broad-beam mode of the SAR. 5.The method of claim 4 wherein transmitting by the transceiver one ormore interrogation pulses in a broad-beam mode of the SAR includestransmitting by the transceiver one or more pulses at a pulse repetitionfrequency in a range of 0.5 Hz to 50 Hz.
 6. The method of claim 1further comprising: receiving by the transceiver a subsequent receivedpulse that includes the subsequent transmitted imaging pulsebackscattered from the ground; encoding by the data processor thesubsequent received pulse to generate a further subsequent transmittedimaging pulse; and transmitting by the transceiver the furthersubsequent transmitted imaging pulse to the ground.
 7. The method ofclaim 6, further comprising: transmitting by the transceiver a secondtransmitted imaging pulse to the ground before transmitting by thetransceiver the subsequent transmitted imaging pulse to the ground,wherein the transmitting of the subsequent transmitted imaging pulsedoes not consecutively follow the transmitting of the first transmittedimaging pulse.
 8. A method of generating by a ground terminal asynthetic aperture radar (SAR) image of the ground, the ground terminalcomprising a transceiver, a data store, and a hardware data processor,the method comprising: receiving by the transceiver an interrogationpulse transmitted by a SAR; transmitting by the transceiver a responseto the interrogation pulse; transmitting by the transceiver positioninformation specifying a ground location; receiving by the transceiveran n^(th) transmitted imaging pulse transmitted by the SAR; storing then^(th) transmitted imaging pulse in the data store; receiving by thetransceiver an m^(th) transmitted imaging pulse transmitted by the SAR,wherein m is greater than n, and wherein a radar return received by theSAR includes the n^(th) transmitted imaging pulse backscattered to theSAR from the ground, and the m^(th) transmitted imaging pulsetransmitted by the SAR includes an encoding of the radar return receivedby the SAR; storing the m^(th) transmitted imaging pulse in the datastore; decoding by the data processor the m^(th) transmitted imagingpulse to regenerate the radar return received by the SAR; and generatinga range line by the data processor for the SAR image of the ground byrange compression of the radar return received by the SAR using then^(th) imaging pulse as a reference function.
 9. The method of claim 8wherein decoding by the data processor the m^(th) transmitted imagingpulse to regenerate the radar return received by the SAR includes:convolutionally decoding by the data processor the m^(th) transmittedimaging pulse to generate a modulated radar return; and demodulating bythe data processor the modulated radar return to regenerate the radarreturn received by the SAR.
 10. The method of claim 8 wherein theencoding of the m^(th) transmitted imaging pulse includes an encryptionof the radar return received by the SAR, and wherein decoding the m^(th)transmitted imaging pulse by the data processor to regenerate the radarreturn received by the SAR includes: decrypting by the data processorthe m^(th) transmitted imaging pulse to generate an encoded radarreturn; convolutionally decoding by the data processor the encoded radarreturn to generate a modulated radar return; and demodulating by thedata processor the modulated radar return to regenerate the radar returnreceived by the SAR.
 11. The method of claim 8 wherein transmitting bythe transceiver position information specifying a ground locationincludes transmitting by the transceiver position information thatincludes the ground location of the ground terminal.
 12. The method ofclaim 8 wherein transmitting by the transceiver position informationspecifying a ground location includes transmitting by the transceiverposition information that includes the center of a desired image groundarea.
 13. A synthetic aperture radar (SAR) imaging system, comprising: aSAR mounted on a SAR platform, the SAR selectively operable in aninterrogation mode to transmit a plurality of interrogation pulses tothe ground; and a ground terminal comprising a ground terminal antennacommunicatively coupled to a transceiver, the transceivercommunicatively coupled to at least one of a ground terminal data storeand a ground terminal data processor, wherein the ground terminal isselectively operable to receive an interrogation pulse and respond bytransmitting position information specifying a ground location to theSAR, and wherein the SAR imaging system, in response to receivingposition information from the ground terminal, generates a SAR image ofthe ground that includes the ground location specified in the positioninformation received in response to the interrogation pulse.
 14. The SARimaging system of claim 13 wherein the SAR platform is selected from agroup consisting of a spaceborne platform and an airborne platform. 15.The SAR imaging system of claim 13 wherein the SAR comprises: a SARantenna that in an imaging mode of operation transmits a plurality oftransmitted imaging pulses to the ground that includes a first and asubsequent transmitted imaging pulse, and receives a plurality ofreceived returns backscattered from the ground that includes a first anda subsequent received return; and a SAR transceiver communicativelycoupled to the SAR antenna, the SAR transceiver communicatively coupledto at least one of a SAR data store and a SAR data processor, the SARdata store communicatively coupled to the SAR data processor, the SARdata store which stores the first received return, and the SAR dataprocessor which encodes the first received return to generate thesubsequent transmitted imaging pulse for transmission by the SAR antennato the ground.
 16. The SAR imaging system of claim 15 wherein the groundterminal transceiver is selectively operable to receive the subsequenttransmitted imaging pulse, and the ground terminal data processordecodes the subsequent transmitted imaging pulse to regenerate the firstreceived return, retrieve a reference function from the ground terminaldata store, and generates a range line by range compression of the firstreceived return using the reference function.
 17. The SAR imaging systemof claim 16 wherein the reference function is, at least in part, derivedfrom the first transmitted imaging pulse.
 18. The SAR imaging system ofclaim 15 wherein the SAR data processor comprises: at least one of aphase-shift keying (PSK) modulator or an amplitude and phase-shiftkeying (APSK) modulator which in operation generates a modulated firstreceived return from the first received return; and a convolutionalencoder which in operation generates an encoded first received returnfrom the modulated first received return.
 19. The SAR imaging system ofclaim 18 wherein the SAR data processor further comprises an encrypterwhich encrypts the encoded first received return.
 20. The SAR imagingsystem of claim 18 wherein the ground terminal data processor comprises:a convolutional decoder which decodes the subsequent transmitted pulse;and at least one of a phase-shift keying (PSK) demodulator or anamplitude and phase-shift keying (APSK) demodulator which in operationregenerates the first received return from the decoded subsequenttransmitted imaging pulse.
 21. The SAR imaging system of claim 13wherein the SAR is selectively operable in a broad-beam mode, and theplurality of interrogation pulses are transmitted while the SAR isoperating in the broad-beam mode.
 22. The SAR imaging system of claim 21wherein the plurality of interrogation pulses are transmitted at a pulserepetition frequency in a range of 0.5 Hz to 50 Hz. 23.-29. (canceled)